The present invention relates generally to the use of polymers in data storage and more specifically to their use as a medium for optical recording and retrieval of data, which is based on light induced modification of their absorption coefficient in the infra-red (IR).
The data storage requirements of new computers and multimedia applications of computers are very large. New and improved, compact, low cost, very high capacity memory devices are required. These memory devices should be able to store many giga-bytes of information, and should randomly retrieve such information at very fast random access speeds demanded by practical applications of modern computing and data processing.
An optical memory offers the possibility of packing binary-stated information into a storage medium at very high density, each binary bit occupying a space only about one wavelength of the writing beam in diameter. This leads to a total capacity of about 1011 bits for a reasonably sized (≈25 cm in diameter) two-dimensional optical storage medium when a writing beam with diameter of about 1 micrometer is used.
At the present three general types of optical recording medium exist, namely ablative recording medium, phase recording medium and amplitude recording medium.
The first type is based on a recording mechanism, which is essentially xe2x80x9cmechanicalxe2x80x9d in nature and utilizes the energy of an adsorbed laser beam to either melt or to actually ablete the material of the recording medium. The result is that dips and even holes are formed which are used to modulate the intensity of the read out laser beam. The current xe2x80x9cwrite once read many timesxe2x80x9d CDs belong to this type.
The second type utilizes light-induced phase changes of the recording media, which modify their index of refraction for the writing mechanism. To this type belong the disclosures of U.S. Pat. No. 4,864,537 to Michl, et al. and U.S. Pat. No. 5,889,756 to Ichihara, et al.
In both cases, the light induced phase changes which affect the refractive index of the medium are accomplished due the local heating of the substrate by the writing laser beam.
The third type refers to photo-induced changes in the absorption coefficient of the recording material.
To this type belong the disclosures in U.S. Pat. No. 5,297,076 to Jefferson, et al. and U.S. Patent No. 5,936,878 to Arsenov, et al.
A common disadvantage of all these types of currently available optical recording and retrieval media is the need for xe2x80x9cchromophoresxe2x80x9d e.g., light absorbing particles dispersed in the matrix of the recording medium which will absorb the energy of the writing beam and enable its interaction with the medium.
This is so because the basic constituents of current optical recording substrates are colorless and hence are transparent to the light of the writing beam in the visible, thus light is not absorbed sufficiently to provide the local amount of the energy dissipation needed for the writing effect to take place.
As a result one has to enhance light absorption at the desired wavelengths by adding light absorbing material into the polymeric matrix which constitute the body of the optical recording medium, as e.g., in U.S. Pat. No. 6,057,020 to Ueno, et al.
Consequently there exist a widely recognized need for an optical recording medium which would overcome this disadvantage of presently known systems as described above.
The present invention provides a recording medium, which belongs to the third type of optical recording media which were mentioned above. It also relates closely to the problem of losses in the connections of optical elements intended for use in middle and far infra-red (IR) bands.
Most if not all adhesives now in use for optical connections are based on organic compounds that have characteristic absorbance peaks in IR (see e.g. in: Liang C., Krimm S., Sutherland G., xe2x80x9cInfrared Spectra of Polymersxe2x80x9d. Journal of Chem. Phys., 25, 543, 1956).
These absorbance peaks in middle and far IR, which are inherent for organic molecules, reduce the optical transmission at the optical contact, thus the elimination of these peaks is desired.
The process of attenuation of absorbance peaks in polymers under thermal exposure was studied by many investigators. Disappearance of absorption at 2012 cmxe2x88x921, which is inherent for ketenimine groups on heating polymethacrylonitrile at 90xc2x0 C. in cyclohexanone solution, was studied in classical works of Grassie and Mc.Neill (Grassie N. Mc.Neill, J. Polymer Science, 33, 171, 1958 and Grassie N., Mc.Neill, J. Polymer Science, 39, 211, 1959 ).
Grassie and Mc.Neill studied different polymers and discovered the effect of disappearing of absorption peaks under heating procedure. They related the effect to the decomposition of polymer and changes in its chemical structure.
The process of peak decay in the IR is attended with changes in the polymer""s color in tile visible (e.g. from colorless for undegraded polymethacrylonitrile to orange-red for the same material heated during 9 hours at 100xc2x0 C.), so the effect is also known as polymer xe2x80x9ccolorationxe2x80x9d.
A technique related to this effect is that of laser marking, see e.g. Tracy Reganall and Mark Wasilenski, xe2x80x9cPigmenting benefits laser marking of thermoplasticsxe2x80x9d in BASF Plastics, 224 October 1998
The absorbance changes due to degradation of polyethylene and epoxy resin films have been studied previously. Fourier transform infrared (FTIR) and Raman spectroscopy were effectively used in the study of polyethylene degradation by Sammon, et al. (Sammon C., Yarwood J., Everall N., xe2x80x9cA FTIR Study of the Effect of Hydrolytic Degradation on the Structure of Thin PET filmsxe2x80x9d, Polymer Degradation and Stability, 67, 149-158, 2000) and in the study of epoxy resin degradation by Farquhrson, et al. (Farquharson S., Bassilakis R., Ditaranto M., Haigis J., Solomon P., Smith W., Ebeling Th., xe2x80x9cMeasurement of thermal degradation in epoxy composites by Fourier transform Raman spectroscopyxe2x80x9d, Proceedings of SPIE, vol. 2072, 319-331, 1994).
We have discovered that changes in the absorbance spectra of a variety of polymeric films, similar to these which were disclosed before, are induced by IR radiation produced by a CO2 laser.
These changes, which are related to the intensity of the irradiating beam and to the duration of the irradiation are used to mark a written zone, which is later subjected to an absorbance measurement for the reading of the recording.
The present invention discloses an optical recording medium, an optical recording system and a method for optical recording and reading in the IR.
In accordance with the present invention there is provided an optical recording medium comprising: (a) a first transparent substrate having an optically flat face; and (b) a recording layer which includes a polymeric film overlaid on the optically flat face of the first transparent substrate.
In accordance with the present invention there is provided an optical recording and retrieval system comprising: (a) an optical recording medium having at least one absorbance band in the spectral region of about 1 micrometer to about 20 micrometer; (b) a first infra-red light source to irradiate on a portion of the optical recording medium; and (c) a mechanism to determine changes in the at least one absorbance peak at the irradiated portion of the optical recording medium.
In accordance with the present invention there is provided a method for optical recording of a plurality of data points data storage comprising the steps of: (a) providing a polymeric layer having at least one absorbance peak in the spectral region from about 1 micrometer to about 20 micrometer, for each data point: (b) producing a light pulse having a power related to a value of said each data point; and (c) changing the at least one absorbance peal of the polymeric layer by irradiating a respective portion of the polymeric layer with the light pulse.
In accordance with the present invention there is provided a method for retrieval of an optical recording comprising the steps of: (a) providing a polymeric, a at least one absorbance peak in the infra-red of a portion whereof has been suppressed by irradiation with a first beam of light; and (b) determination the amount of said suppression of the at least one absorbance peak at the at least a portion of the polymeric layer.
It is the object of the present invention to provide an optical recording medium that is based on simple, common, and cheap polymers.
It is another object of the present invention to provide an optical recording medium which does not need the incorporation of light absorbing dyes.
It is still another object of the present invention to provide an optical recording medium in which the writing beam uses little power.
It is yet another object of the present invention to provide an optical recording medium with high recording density.